1. Field of the Invention
The present invention relates to thermoplastic biodegradable resins having a number average molecular weight of 25,000 to 45,000 and to method for making the same.
2. Discussion of the Background
Recently, environmental pollution problems have taken on increasing worldwide concern, especially because the plastics used for all purposes are not degradable. Such undegradable plastics therefore contribute to the destruction of the natural environment. Attention is therefore now being directed to ways in which the problem of the disposal of waste undegradable plastics can be solved.
An aliphatic polyester is known which is biodegradable (Journal of Macromol, SCI-Chem., A-23(3), 1986, 393-409). It has a variety of uses in medical, agricultural, fishery packaging material and other applications are being developed. However, because the conventional aliphatic polyester has a low melting point and a high melt index, because of the structure of the main chain and crystallinity, and has low heat-resistance and unsatisfactory mechanical properties, the utility of this polymer material has been limited. (R&D Evaluation Report No. 47 Chart 3).
Japanese laid-open patent No. 4-189822 and 4-189823 disclose a process for the preparation of aliphatic polyester. But here, because the polyester has a low melting point and a molecular weight of less than 20,000, the polyester is not used as industrial material. A method is known in which an isocyanate is introduced into the polyester.
However, because the method introduces isocyanate, it is not desirable to use the modified substance as an industrial material. Isocyanate is harmful to the human body, and further it is hard to cleave the ester linkage in comparison to the polyester into which the isocyanate is not introduced. A need therefore continues to exist for a biodegradable polyester of improved mechanical properties and heat resistance.